Up to now, in some of image forming apparatuses such as a facsimile apparatus and a copying machine, a dot-like image is formed on a sheet by placing an ink ribbon on a surface of the sheet and striking the ink ribbon with dot pins of a recording head, or an image is formed on a sheet by discharging ink drops to the sheet from a nozzle of an ink jet head as in an ink jet type image forming apparatus. In these image forming apparatuses, since a gap between a face of a recording head and a sheet significantly affects print quality, there have been devised various image forming apparatuses each of which is provided with a mechanism capable of adjusting the gap between the face of the recording head and the sheet according to a thickness of the sheet.
For example, in JP 5-104817A, an image forming apparatus is disclosed in which: a carriage mounted with a dot pin type recording head is attached on a guide shaft positioned in parallel with a platen and laterally slidably; and an eccentric shaft decentered with respect to an axis of this guide shaft is pivoted by a contact and separation mechanism, thereby making it possible to adjust a distance (gap) between a head face of the recording head and a sheet.
In addition, in JP 11-348373A, an image forming apparatus is disclosed in which a gap adjustment member is positioned displaceably in a position opposed to an ink jet type recording head across a sheet conveying path.
With these image forming apparatuses, for example, an operator executes gap adjustment by selecting and inputting a sheet type to be used with an input unit of a computer or the like according to a thickness of a sheet on which an image is to be printed and actuating a drive motor of the contact and separation mechanism (gap adjustment member) in response to an input signal from the input unit, or the operator executes manual adjustment by actuating the contact and separation mechanism (gap adjustment member) with a manual lever. In the above-related arts, the carriage or the gap adjustment member is actuated such that the face of the recording head moves away from or close to the surface of the sheet in a parallel manner.
On the other hand, in image forming apparatuses disclosed in JP 8-300768A and JP 10-250184A, one end of a carriage mounted with a recording head is slidably and pivotably attached to a guide shaft with a round shaft shape, a lever provided on the other end side of the carriage is attached to a guide plate, which guides the other end side of the carriage, pivotably around a spindle, an eccentric cam is provided around the spindle, and a projection is provided in a hold portion of the lever. Further, the guide plate is nipped by the eccentric cam and the projection vertically. According to selection of a pivotal position of the lever, in a state in which the guide plate is nipped by a major diameter portion of the eccentric cam and the projection, a gap between a face of the recording head and a sheet is set to be large as the carriage pivots around the spindle. On the contrary, in a state in which the guide plate is nipped by a minor diameter of the eccentric cam and the projection, the gap is set to be small. The lever is manually operated by an operator.
However, in order to select and input a sheet type to be used with the input unit of the computer or the like to adjust a gap to a predetermined amount as described above, a complicated electric mechanism member is required. In addition, in the image forming apparatus in which the lever is operated manually, unless a user knows that the lever is manually operated, the user cannot perform gap adjustment and an image is formed with an inappropriate gap with respect to a thickness of a sheet, which provides inconveniences such as a waste of sheets and a waste of time.
In addition, in the case of the ink jet type image forming apparatus, there are provided a chip type in which a recording head is directly attached to a carriage which is reciprocatingly movable in a direction crossing a conveying direction of a recording medium, and another type in which a recording head unit is detachably mounted on a carriage and an ink cartridge (ink tank) is further detachably mounted thereon. In both the types, positioning of the recording head (recording head unit) to the carriage significantly affects print quality.
For example, in JP 2001-18416A, a carriage is provided with a receiving portion including four side plates and a bottom plate to provide upper open end, and a covering portion is provided on one side of the open end. While one end of the recording head unit is inserted below the covering portion, the other end thereof is pivoted downward to be dropped into the receiving portion, and the other end side of the recording head unit is pressed by a pressing lever provided in the other side plate of the carriage for positioning the recording head.
However, in the related art described in JP 2001-18416A, a nozzle portion in the recording head unit is faced with a nozzle placing portion which is constituted by the side plate having a cut away portion and vertically provided in a direction perpendicular to the pair of parallel side plates. Therefore, there is a problem in that a pressing force of the pressing lever does not act on the positioning of the nozzle portion at all and the recording head unit tends to be unsteady with respect to the receiving portion of the carriage and also tends to positionally deviate.
Besides, since the recording head unit is inserted obliquely downward with respect to the receiving portion as described above, there is also a problem in that mounting work thereof is not easy.
Moreover, as a flow of a basic operation of an ink jet printer of this type, first, after moving the carriage to a retract position at the time of sheet feeding, a sheet is fed so as to reach a position directly below a position to which the recording head is moved. Thereafter, movement of the carriage and conveyance of sheets are alternately performed according to a print command or a paper feed command. Thereafter, when the ink jet printer receives a paper supply command following the paper feed command without receiving the print command, after the carriage is moved to the retract position at the time of sheet feeding, feeding of a sheet is performed according to the paper feed command, and sheet discharge is performed such that the sheet is guided to a sheet discharge portion through the position directly below the position of the movement of the recording head. In addition, after printing of one page is finished, in order to detect ink empty, it is necessary to move the carriage to a position where an ink empty detection sensor is positioned. Alternatively, after printing of all pages is finished, it is necessary to move the carriage to a head protection position.
Consequently, in the above-mentioned image forming apparatus, many positioning operations are performed for moving and stopping the carriage, supplying and discharging sheets, and the like. Thus, there is a waste of movements in operations related to printing which are performed from supply to discharge of sheets, and speeding-up of printing cannot be realized easily.
For example, a reference for determination on stop of the carriage for reversing the carriage from deceleration to acceleration during printing is substantially fixed without any change from a reference for moving the carriage to a retract position at the time of sheet feeding and discharge, a head protection position, a flushing position, or the like to determine that the carriage stops. Thus, a time required for an entire reciprocal movement of the carriage is never shortened.
In addition, prior to discharge of a sheet according to a sheet discharge command, it is necessary to move the carriage to the retract position for sheet feeding or discharge. Thus, time is wasted by an amount of time required for moving the carriage from a print end position to the retract position at the time of sheet feeding and discharge, and sheet discharge cannot be performed promptly.
The present invention has been established in order to solve these problems, and it is an object of the present invention to provide an image forming apparatus which makes it possible to adjust a size of a gap between a recording head and a recording medium by a simple operation of only moving a carriage in a reciprocating scanning direction.
Another object of the present invention is to provide an image forming apparatus which can, with a simple structure, realize accurate positioning of a recording head unit to a carriage and make the recording head unit detachably attachable.
Still another object of the present invention is to provide an image forming apparatus which can perform high speed printing, and to provide a computer program for realizing operations of such an apparatus.